Low-molecular weight liquid crystal compounds are typically useful as display materials for flat panel displays and have been widely used in TN mode liquid crystal displays, e.g., digital watches, and STN mode liquid crystal displays, e.g., lap-top computers and lap-top word processors. In these displays, "on" (transparent) and "off" (non-transparent) states are controlled by utilizing the phenomenon that the light transmitted through the liquid crystal layer changes its plane of polarization with a voltage applied. Therefore, the devices must have a pair of polarizing sheets in which the liquid crystal layer is sandwiched. Further, an orientation treatment is essentially needed for the liquid crystal layer to have specific orientations.
In recent years, hybrid type liquid crystal display elements have been proposed, in which a low-molecular weight liquid crystal compound is supported in a polymeric binder by dispersion or encapsulization as disclosed, e.g., in U.S. Pat. No. 4,435,047, JP-W-58-501631 (the term JP-W as used herein means an "unexamined published international patent application"), JP-W-61-502128, JP-A-62-2231 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-A-64-62615, JP-A-1-312527, and JP-A-2-55787.
More specifically, the composite film disclosed in JP-B-3-52843 (the term "JP-B" as used herein means an "examined published Japanese patent application") and JP-W-58-501631 comprises polyvinyl alcohol having dispersed therein microcapsules of a nematic liquid crystal material. With no voltage applied, the nematic liquid crystal in the microcapsules are oriented along the capsule wall and therefore scatters incident light. With voltage applied, the nematic liquid crystal is oriented in the direction of the applied electrical field and therefore transmits incident light. JP-W-61-502128, JP-A-62-2231, and JP-A-64-62615 teach a process for preparing a liquid crystal element comprising a polymeric binder having dispersed therein liquid crystal droplets, comprising polymerizing a polymerizable composition comprising a polymerizable monomer (e.g., an epoxy compound) and a low-molecular weight liquid crystal material by ultraviolet light or heat application to cause phase separation. JP-A-2-55787 proposes to raise the proportion of the low-molecular weight liquid crystal material to be used in the above-mentioned photopolymerizable composition thereby to reduce a driving voltage. JP-A-63-43993 and JP-A-63-137211 propose to produce a liquid crystal element, in which liquid crystal droplets are maintained in a high-molecular weight material, by dissolving a low-molecular weight liquid crystal material and a high-molecular weight material in a common organic solvent, and then evaporating the organic solvent.
In this type of displays, since switching between "on" and "off" states is controlled by utilizing the phenomenon that the light transmitted changes its scattering state on application of voltage, provision of polarizing sheets, which have been inevitable in conventional TN and STN mode liquid crystal display elements, is unnecessary. Display devices using the hybrid type elements therefore realize brighter display than with the elements having polarizing sheets. Further, no orientation film being used, the steps involved therefor, such as printing, baking and rubbing of an orientation film, are unnecessary, which is expected to realize widening of the area of an element and to bring about reduction of cost.
However, since the conventional hybrid type display elements comprising a polymeric binder having supported therein a low-molecular weight liquid crystal compound need a high voltage of from several tens to 100 Vrms for driving the liquid crystal-polymer composite film, they have a narrow range of application. That is, they are difficult to apply to an active matrix display system using TFT or MIM, still less a time-sharing driving system, and have found use only as light controlling elements of simple structure (light controlling glass).
Further, they lack in steepness in the voltage-transmission curve and have no memory function, such as hysteresis characteristics, which disadvantages have made it virtually impossible to apply them to wide-area display elements.
Furthermore, they have insufficient heat resistance and readily undergo changes of the phase separation state with heat, resulting in changes of electro-optical characteristics.
In addition, the conventional liquid crystal-polymer composite film exhibits poor adhesion to a substrate, particularly a flexible substrate, such as a polyethylene terephthalate film. An attempt of providing an adhesive layer or using an adhesive may be effective to improve adhesion but causes deterioration of the electro-optical characteristics.